Si3 N4 Ceramic densified using Sc2 O3 and SiO2

ABSTRACT

At least 1 mole % SiO 2  and 1/2 mole % Sc 2  O 3  are used as a densification aid to provide a polyphase silicon nitride ceramic having high creep strength. The composition is within the Si 3  N 4  - Si 2  N 2  O tie line, the Si 3  N 4  - Sc 2  Si 2  O 7  tie line, and the Sc 2  Si 2  O 7  - Si 2  N 2  O tie line in the ternary phase diagram for the Si 3  N 4 , SiO 2 , Sc 2  O 3  system. The phases in the sintered ceramic are Sc 2  Si 2  O 7 , and Si 3  N 2  O, and at least 50 mole % Si 3  N 4 .

STATEMENT OF GOVERNMENT INTEREST

This invention was made under a contract with the Department of Navy.

BACKGROUND OF THE INVENTION

This invention relates to the field of ceramics, and particularly to Si₃ N₄ ceramics.

Silicon nitride ceramics encompass a family of compositions in which Si₃ N₄ constitutes an important ingredient. Considerable effort has been expended and is being expended to develop ceramics which have useful structural properties at high temperatures. In particular, there is a continuing need for Si₃ N₄ ceramics which have high oxidation and creep resistance.

Si₃ N₄ ceramics are produced by compacting Si₃ N₄ powder and then sintering or hot pressing the powder at high temperature. Very pure Si₃ N₄ powder cannot be successfully sintered or hot pressed and consequently a densification aid must be added to the Si₃ N₄ powder in order to obtain dense Si₃ N₄. The densification aid reacts with SiO₂ on the surface of the Si₃ N₄ particles, with the Si₃ N₄, and with impurities in the powder to form a eutectic liquid during sintering. The liquid promotes densification of the powder through a solution-reprecipitation mechanism.

Upon cooling, the liquid freezes and forms secondary phases. The number, chemistry, and content of the secondary phases depend on the constituents in the starting powder and the phase relationship of the composite system. Both non-equilibrium (nitrogen glasses) and crystalline phases are observed. Thus, Si₃ N₄ alloys are polyphase materials. As expected, the secondary phases significantly influence the properties of the ceramic, unfortunately by decreasing the ceramic's creep and oxidation strength at high temperatures.

One approach to overcoming the harmful effect of the secondary phase is to reduce the amount of secondary phase in the ceramic. U.S. patent application Ser. No. 266,244, filed May 22, 1981 by F. F. Lange and D. R. Clarke, describe a method of drawing out some of the glassy phase by a post-fabrication oxidation treatment.

Another approach is to use ingredients which produce a secondary phase which has improved high temperature properties. This second approach is attempted in the method described in U.S. Pat. No. 4,046,580, wherein Y₂ O₃ is used as a densification aid.

Currently, MgO is the densification aid used in most commercial Si₃ N₄ ceramics. Other known aids include ZrO₂, Al₂ O₃, BeO, and the rare earth oxides Y₂ O₃, CeO₂, Ce₂ O₃, Nd₂ O₃, and La₂ O₃. Unfortunately, all these aids form intergranular phases which are either viscous glasses at high temperature (thus reducing the ceramic's creep strength) or are metal-silicon-oxynitride compounds (sometimes causing cracking by expanding during oxidation).

SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved Si₃ N₄ ceramic.

It is an object of the invention to provide a Si₃ N₄ ceramic having improved creep strength and improved oxidation resistance.

It is an object of the invention to provide a densification aid for use in fabricating Si₃ N₄ ceramic having improved high temperature properties.

According to the invention, Sc₂ O₃ and SiO₂ are used as a densification aid during the sintering of Si₃ N₄ powder. The amount of Sc₂ O₃ and SiO₂ used is selected to be within the composition defined by a triangle formed by the Si₃ N₄ -Si₂ N₂ O tie line, the Si₃ N₄ -Sc₂ Si₂ O₇ tie line, and the Sc₂ Si₂ O₇ -Si₂ N₂ O tie line in the ternary phase diagram for the Si₃ N₄, SiO₂, Sc₂ O₃ system. No quaternary compound is formed within this composition range so that problems such as cracking caused by the expansion on oxidation of quaternary compounds (metal-silicon-oxynitrides) are avoided.

The powders used to form the ceramic should include 25 to 98.5 mole % Si₃ N₄, 1 to 50 mole % SiO₂, and 0.5 to 25 mole % Sc₂ O₃. The proportion of these powders is selected so that the ceramic, after sintering, will contain at least 50 mole % Si₃ N₄ with the balance being the phases Si₂ N₂ O and Sc₂ Si₂ O₇. It must be recognized that most Si₃ N₄ powders already contain some SiO₂ and this must be taken into account.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a ternary phase diagram in mole % for the Si₃ N₄, SiO₂, Sc₂ O₃ system for samples hot-pressed in graphite dies at 1600° to 1800° C.

DESCRIPTION OF THE PREFERRED EMBODIMENT

It has been discovered that Sc₂ O₃ and SiO₂ can be used as a densification aid for sintering Si₃ N₄ powder in such a way that no quaternary compounds (scandium-silicon-oxynitrides) are formed. Such quaternary compounds could cause problems such as cracking caused by expansion on oxidation.

To determine the phases formed during sintering, mixtures of Si₃ N₄, Sc₂ O₃, and SiO₂ powders were hot-pressed in graphite dies at temperatures from 1600° to 1800° C. The sintered samples were ground into powder and x-ray analyzed to identify the constituent phases and to construct the ternary phase diagram shown in FIG. 1. No quaternary compounds were observed. The compounds observed were Si₃ N₄, Si₂ N₂ O, and Sc₂ Si₂ O₇. Their tie lines were the Si₃ N₄ -Si₂ N₂ O tie line 2, the Si₃ N₄ -Sc₂ Si₂ O₇ tie line 4, and the Sc₂ Si₂ O₇ -Si₂ N₂ O tie line 6.

This invention is limited to compositions within tie lines 2, 4, and 6, in order to insure that only three compounds (Si₃ N₄, Sc₂ Si₂ O₇, and Si₂ N₂ O) can exist in the sintered ceramic. These three compounds are compatible with each other, and additionally the Sc₂ Si₂ O₇ is compatible with SiO₂ which is the oxidation product of Si₃ N₄. In order to obtain the intrinsic properties of Si₃ N₄ such as low thermal expansion and moderate elastic modulus, the composition is also limited to a minimum of 50 mole % Si₃ N₄ in the sintered ceramic, as shown by dashed line 8.

A minimum of about 0.5 mole % Sc₂ O₃ is required in the mixture of powders to serve as a densification aid as shown by dashed line 10. This minimum amount of Sc₂ O₃ intersects the Si₃ N₄ -Sc₂ Si₂ O₇ tie line 4 at about 1 mole % SiO₂, thus setting the minimum amount of SiO₂ required as a densification aid in the Si₃ N₄ rich corner of the phase diagram.

When Si₃ N₄, SiO₂, and Sc₂ O₃ powders are mixed and sintered at 1600°-1800° C., they react to form a sintered ceramic according to the following relationship:

    xSi.sub.3 N.sub.4 +ySiO.sub.2 +(1-x-y)Sc.sub.2 O.sub.3 →aSi.sub.3 N.sub.4 +bSi.sub.2 N.sub.2 O+cSc.sub.2 Si.sub.2 O.sub.7.

Point A in FIG. 1 is a composition using the minimum amount of Sc₂ O₃ powder (0.5 mole %) and providing a sintered ceramic with the minimum amount of Si₃ N₄ (50 mole %). Using the above relationship, the mixture of powders required at Point A is: 74 mole % Si₃ N₄, 25.5 mole % SiO₂, and 0.5 mole % Sc₂ O₃. After sintering this mixture of powders, the sintered ceramic should have a composition of: 50 mole % Si₃ N₄, 49.5 mole % Si₂ N₂ O, and 0.5 mole % Sc₂ Si₂ O₇.

Point B in FIG. 1 is a composition using the minimum amount of Si₃ N₄ powder (25 mole %) and providing a sintered ceramic with the minimum amount of Si₃ N₄ (50 mole %). Using the above relationship, the mixture of powders required at point B is: 25 mole % Si₃ N₄, 50 mole % SiO₂, and 25 mole % Sc₂ O₃. After sintering this mixture of powders, the sintered ceramic should have a composition of: 50 mole % Si₃ N₄ and 50 mole % Sc₂ Si₂ O₇.

Point C in FIG. 1 is a composition using the minimum amounts of Sc₂ O₃ and SiO₂ powders. Using the above relationship, the mixture of powders at point C is: 0.5 mole % Sc₂ O₃, 1.0 mole % SiO₂, and 98.5 mole % Si₃ N₄. After sintering this mixture of powders, the sintered ceramic should have a composition of: 99.5 mole % Si₃ N₄ and 0.5 mole % Sc₂ Si₂ O₇.

In order to determine the properties of a ceramic made according to the present invention, a mixture of powders containing 74 mole % Si₃ N₄, 18 mole % SiO₂, and 8 mole % Sc₂ O₃ (Composition D in FIG. 1) was prepared. The powders were milled in methanol with WC grinding media, dried, and then hot-pressed at 29 M Pa in graphite dies. Table I lists the hot-pressing conditions, densities achieved, and the observed phases, the quantities of which (according to the previously mentioned relationship) should be: 4.8 mole % Si₂ N₂ O, 9.5 mole % Sc₂ Si₂ O₇, and the balance substantially Si₃ N₄. As indicated, the theoretical density of 3.21 g cm⁻³ could be achieved in 2 hours at 1800° C., but a 4 hour hold was required to nearly complete the α to β phase transformation of the Si₃ N₄. The sluggish densification and transformation kinetics of this composition as evidenced by these results relative to other Si₃ N₄ systems would be indicative of slow kinetics in the solution/reprecipitation reaction which would imply good high temperature creep resistance also.

                  TABLE I.                                                         ______________________________________                                         Densification Results                                                          Hot-pressing                                                                            Density                                                               conditions                                                                              (g/cm.sup.3)                                                                               Phases                                                    ______________________________________                                         1750° C./2 h                                                                     2.62     α-Si.sub.3 N.sub.4, β-Si.sub.3 N.sub.4,                             Sc.sub.2 Si.sub.2 O.sub.7, Si.sub.2 N.sub.2 O                1800° C./1 h                                                                     3.14     β-Si.sub.3 N.sub.4, α-Si.sub.3 N.sub.4,                             Sc.sub.2 Si.sub.2 O.sub.7, Si.sub.2 N.sub.2 O                1800° C./2 h                                                                     3.21     β-Si.sub.3 N.sub.4, Sc.sub.2 Si.sub.2 O.sub.7,                            α-Si.sub.3 N.sub.4, Si.sub.2 N.sub.2 O                 1800° C./4 h                                                                     3.21     β-Si.sub.3 N.sub.4, Sc.sub.2 Si.sub.2 O.sub.7,                            α-Si.sub.3 N.sub.4 (Tr), Si.sub.2 N.sub.2              ______________________________________                                                           O                                                       

Specimens for preliminary oxidation and compressive creep experiments were diamond cut and ground from the Composition D billet hot pressed at 1800° C. for 4 hours.

After 285 hours of oxidation in air at 1400° C., the specimen had a slight, glassy patina and had gained 4.4×10⁻³ kg·m⁻². This weight gain is much lower than for commercially available Si₃ N₄. Sc₂ Si₂ O₇ is compatible with Sc₂ O₃, Si₂ N₂ O, and SiO₂ so that destructive interaction on oxidation is avoided and the protective oxidation surface layers have good integrity and remain thin as confirmed by these oxidation tests.

Compressive creep measurements were conducted in air at 1400° C. using the method described by F. F. Lange, D. R. Clarke, and B. I. Davis in "Compressive Creep of Si₃ N₄ /Magnesium Oxide Alloys, Part I, Effect of Composition", The Journal of Material Science, Vol. 15, (1980). The specimen's creep resistance was 1 to 2 orders of magnitude higher than for the Si₃ N₄ /MgO material examined in the above mentioned article.

The microstructure of the Composition D Si₃ N₄ /Sc₂ O₃ material was examined using high-resolution electron microscopy and analytical electron microscopy. Gains of Si₃ N₄ were surrounded by grains of the secondary phase crystalline Sc₂ Si₂ O₇. There was an extremely thin (approximately 1 nm), continuous intergranular phase. In addition, a few large pockets of glass were seen, suggesting that the volume fraction of noncrystalline phase present was small.

Numerous variations and modifications can be made without departing from the invention. For example, the ceramic can include minor impurities or additives which are included for their known effect. Accordingly, it should be clearly understood that the form of the invention described above and shown in the accompanying drawings. is illustrative only and is not intended to limit the scope of the invention. 

What is claimed is:
 1. A ceramic having good oxidation and creep resistance at 1400° C., said ceramic having a composition within the following tie lines in the ternary phase diagram:Si₃ N₄ -Si₂ N₂ O tie line; Si₃ N₄ -Sc₂ Si₂ O₇ tie line; Sc₂ Si₂ O₇ -Si₂ N₂ O; and containing more than 50 mole % Si₃ N₄ and at least 0.5 mole % Sc₂ Si₂ O₇.
 2. A densified, polyphase ceramic having good oxidation and creep resistance at 1400° C., said ceramic consisting essentially of:4.8 mole % Si₂ N₂ O; 9.5 mole % Sc₂ Si₂ O₇ ; and balance substantially Si₃ N₄.
 3. An improvement in a method of forming a densified polyphase ceramic, said method having the steps of:providing over 25 mole % Si₃ N₄ powder; providing a densification aid; mixing said Si₃ N₄ with said densification aid; and sintering the mixture;said improvement being characterized in that said step of providing a densification aid comprises: providing from 1 to 50 mole % SiO₂ powder; and from 1/2 to 25 mole % Sc₂ O₃ powder, whereby the oxidation and creep resistance at 1400° C. of said ceramic are increased. 